The GNU system can handle most input/output operations on many different
devices and objects in terms of a few file primitives - read,
write and lseek. However, most devices also have a few
peculiar operations which do not fit into this model. Such as:
Changing the character font used on a terminal.
Telling a magnetic tape system to rewind or fast forward. (Since they
cannot move in byte increments, lseek is inapplicable).
Ejecting a disk from a drive.
Playing an audio track from a CD-ROM drive.
Maintaining routing tables for a network.
Although some such objects such as sockets and terminals
1 have special functions of their own, it would
not be practical to create functions for all these cases.
Instead these minor operations, known as IOCTLs, are assigned code
numbers and multiplexed through the ioctl function, defined in
sys/ioctl.h. The code numbers themselves are defined in many
— Function: int ioctl (int filedes, int command, ...)
The ioctl function performs the generic I/O operation
command on filedes.
A third argument is usually present, either a single number or a pointer
to a structure. The meaning of this argument, the returned value, and
any error codes depends upon the command used. Often -1 is
returned for a failure.
On some systems, IOCTLs used by different devices share the same numbers.
Thus, although use of an inappropriate IOCTL usually only produces
an error, you should not attempt to use device-specific IOCTLs on an
Most IOCTLs are OS-specific and/or only used in special system utilities,
and are thus beyond the scope of this document. For an example of the use
of an IOCTL, see Out-of-Band Data.
 Actually, the terminal-specific functions are implemented with
IOCTLs on many platforms.
Published under the terms of the GNU General Public License